Air flow monitoring enclosure intrusion alarm having retarded flowmeter



NOV. 29, p DAVEY AIR FLOW MONITORING ENCLOSURE INTRUSION ALARM HAVING RETARDED FLOWMETER Filed April 10, 1964 /7 6-3 fw? F G .6 lNvgNgORi/iy 4 M M P5752 4 62 BY 8 wax W irraeWE/S United States Patent 3,289,12 AIR FLOW MONHTORTNG ENCLOSURE INTRU- SIGN ALARM HAVING RETARDED FLOW- ItiiETER Peter G. Davey, 2125 Hearst Ave., Berkeley, Calif. Filed Apr. 10, 1964, Ser. No. 358,710 2 Claims. (Cl. 340239) The present invention is directed to the provision of an improved alarm system for protecting a given volume from unauthorized access, and operates upon the basis of pressure change in such volumes as produced by the access.

The improved alarm system of the present invention provides for the slight pressurization or partial evacuation of a volume to be protected. Flow of air into or out of the volume is then monitored to indicate changes in openings thereto as result from entry to the volume. The invention is applicable to the protection of volumes ranging in size from a few cubic feet, as in a museum showcase, through several thousand cubic feet of vaults or warehousing. Although it is known to employ volume protection devices and systems utilizing means such as ultrasonic or radio standing waves, it is also well known that substantial equipment is necessary in this type of protection system, and also that certain portions of the volume may well be unprotected because of blocking of the waves from such portion. An even more widespread type of protection system employs the utilization of a grid of conductors, as in the form of metallic tape placed upon windows, doors, and the like, so that upon breaking of this tape for unauthorized entry an electrical circuit is directed to thereby produce an alarm.

The present invention operates upon an entirely different basis to provide protection of a given volume. The pressurization or partial evacuation of the volume to be protected affords a highly advantageous manner of determining unauthorized access thereto. The opening of an aperture in the walls of the volume sufficiently to enable access to the volume will produce a substantial change in the pressure therein, and the present invention provides for the detection of this pressure change by means of a simple solid-state thermal flowmeter, which is applicable to operate any type of normal electrical alarm. Investigation has shown that the normal degree of sealing of a wide variety of enclosures is quite adequate for the requirements of this invention without extensive modifications. Although the present invention is directed to the protection of closed volumes, small leaks as may be found around conventional doors, windows, or the like, are not limiting upon the present invention, since the pneumatic resistance of apertures varies inversely with the fourth power of the diameter thereof. The invention is applicable to protection of substantially sealed enclosures having only small apertures or cracks therein. Consequently, a room, or the like, may have a total leakage to the exterior of the order of 30 or 40 square inches divided among openings about Windows and doors, and yet only slight pressurization of the volume, in accordance with the present invention, is quite adequate to produce a readily detectable pressure change upon even relatively small opening of a door or window to the room. By the utilization of a thermal type flowmeter only a minute pressure change is required to produce a readily detectable signal. Thus a one-half inch opening of a door to a thousand cubic foot volume pressurized to about .025 inch of water is readily detected with this invention.

The alarm system of the present invention is intrinsically fail-safe, inasmuch as any tempering with the small pressurizing apparatus or evacuating apparatus employed herewith will naturally produce a variation in the pres- "ice sure of the protected volume, which is then at once detected by the flowmeter type device utilized herewith, so as to actuate the alarm circuit. It is particularly noted that the alarm produced by the present invention is given substantially instantaneously, inasmuch as air pressure variations occur within a second or two after substantial breaking of the seal of a protected volume. It is quite easy for the present invention to be employed to produce a permanent security record of the volume under protection by the utilization of a standard electrical pen record-er in connection with the pressure variation indicating apparatus hereof. Furthermore, the present invention operates to produce an alarm, no matter how cautiously or deliberately an intruder may attempt to gain access to the protected volume, inasmuch as the blower of the invention maintains a steady pressure or vacuum in the protected volume, and this pressurized condition will be varied by the unauthorized opening of any access medium to the volume.

The present invention, in brief, comprises a small, preferably electrically-driven blower, which may, for example, be connected to a duct in a wall of the volume to be protected. The blower may either pressurize or evacuate the volume, and only a very slight pressure differential from normal need be maintained. A highly sensitive flowmeter is utilized to detect the flow of air through a separate small duct into or out of the protected volume, depending upon whether the volume is pressurized or evacuated. Signals produced by variations in the flow through this flowmeter are employed in a simple, electrical circuit to produce an output signal which may be utilized to actuate any type of conventional type of alarm or recording system desired. The invention is furthermore applicable under conditions wherein a variable external air pressure may be experienced as, for example, in a store or Warehause against which the wind may blow. A difierential type of air flow system is provided by the present invention to produce the same alarm signal whether or not external air pressure varies. The differential system may be employed either by dividing the protecting volume in half, or by utilizing a capacitive effect in a small air-pressure storage unit which may be combined with other elements of the present invention.

The present invention utilizes a detector unit employing a heated thermistor element connected in a bridge circuit with a similar thermistor employed as the temperature reference. The first thermistor is mounted in position to be cooled by air flow passing through the device, and the temperature reference is located out of the air flow, so that there is produced a relatively large electrical signal for air flows of considerably less than one cubic centimeter per minute. With the maintenance of a pressure difierence between two ends of a how tube containing the first thermistor, pressure variations down to of an inch of water can be reliably measured. This detector unit, including the flow tube, thermistors, and bridge components, can be readily fitted into a one-inch cube and requires only about milliwatts of electrical power from a four-volt power supply. The detector unit is extremely inexpensive, reliable and accurate. Together with this detector unit, there is employed a current measuring device which then provides a pressure differential reading. In conjunction with this measuring device, there is employed means for tripping an alarm system when the pressure varies from a preadjusted normal range. It is possible also to utilize transistorized circuitry for energization of the alarm system, and under certain circumstances it is advisable to also employ some type of delay device, so that an operator may leave the protected volume after switching on the system without actuating the alarm. A surprisingly small blower is quite adequate for purposes of the present invention, inasmuch as the flowmeter is sensitive enough to detect very slight pressure changes. It is herein only contemplated that a very small pressure differential shall be produced by the blower of this invention, and thus for a museum showcase, for example, an 18 watt fan may be utilized. For larger volumes to be protected, it has been found that a moderately powerful blower of about 100 watts per thousand square feet of floor area is quite satisfactory; however, it is to be noted that the actual blower requirements depend at least in part upon the air leakage in any particular installation.

The present invention is illustrated in connection with particular preferred embodiments thereof in the accompanying drawing, wherein:

FIGURE 1 is a schematic illustration of the installation of a blower and detector unit in a single room for protection thereof;

FIGURE 2 is a schematic sectional view of a flowmeter adapted for utilization as the detector unit of the present invention;

FIGURE 3 is a schematic circuit diagram of a measuring circuit applicable for use with the flow meter of FIG- URE 2;

FIGURE 4 is a graph of output signal vs. air flow for the detector of FIGURE 2, as employed in the circuit of FIGURE 3;

FIGURE 5 is a schematic illustration of a differential air flow alarm system, particularly applicable to buildings or relatively large volumes possibly subjected to external wind pressure; and

FIGURE 6 is a schematic illustration of an alternative differential air flow alarm system.

Considering now the invention in general, reference is made to FIGURE 1 of the drawings, wherein there is shown an enclosure 11 defining a volume 12 therein. This enclosure may comprise, for example, a showcase, a store window, or an individual room in a building. Whatever the type of enclosure, there will be provided certain openings thereto, and in the instance wherein the volume to be protected is a single room, there may be provided a door 13 and windows 14. These openings are closed during operation of the invention to thus provide a substantially sealed enclosure with only normal air leaks about the door and windows. In accordance with the present invention, there is disposed within the volume 12 a small air pump or blower 16-, which is illustrated as exhausting through a duct 17 extending from the interior to the exterior of the enclosure through a wall thereof. There will consequently be produced a flow of air from the interior to the exterior of the volume 12, and with the door 13 and the windows 14 closed, there will consequently be produced at least a slight pressure differential between the interior and the exterior of the volume. Even though there may exist certain air leakage paths, as for example about the door and windows, there will yet be produced at least some pressure differential by the action of the air pump or blower 16. The present invention operates to employ this slight pressure differential as a means for determining or recording continue-d closure of the volume, aside from the normal leakage paths thereto. This detection or recording is accomplished herein by the provision of a flowmeter 18 including a small tube 19 extending from the interior of the volume 12 wherein the flowmeter is disposed, to the exterior thereof through a wall of the enclosure 11. The pressure differential existing between the interior of the enclosure and the exterior will consequently cause a flow of air through the flowrneter 18. It is to be appreciated that the air pump or blower 16 may be mounted either to pressurize or evacuate the enclosure, and in either instance there will be produced a flow of air through the flowme-ter 18. This flow of air will be maintained substantially constant with the access means 13 and 14 closed. However, the flow of air will be materially varied by even a relatively small opening of either the windows 14 or the door 13. Variation in the flow of air through the flowmeter 18 r is herein employed to produce an electrical signal, which may then be further utilized to actuate an alarm system or recording means. As briefly stated above, leakage paths as about the door or Windows to the protected volume 12 do not materially affect operation of the present invention. Assuming laminar flow of air, the flow is inversely proportional to the fourth power of the diameter of an opening to the volume. Consequently, small openings as may exist about a door, for example, will cause a relatively small leakage of air into or out of the protected volume. However, a relatively small opening of the door, as for example one-half inch, will produce a very measurable difference in pressure within the volume, so as to thereby produce a measurable difference in pressure differential across the flowmeter so that an electrical signal produced thereby is adequate to actuate an alarm or recording mechanism.

Before proceeding further with discussion of the present invention, attention is directed to the type of flowmeter which may be utilized in the present invention. There is illustrated in FIGURE 2, a typical flowmeter having inlet and outlet tubes 21 and 22 with a central apertured plate 23 therebetween. As illustrated, each of the pipes may be flanged at the facing ends thereof, and O-rings, or the like 24, disposed between these flanged ends and the central plate 23. A flow of air in through the pipe 21 will consequently pass through the small aperture in the plate 23 and out the pipe 22. Disposed in the path of this air flow is a thermistor 26. It is well known that a property of thermistors is a variation in resistance thereof with temperature. In addition to this detecting thermistor there is employed a reference thermistor 27, disposed out of the air flow path through the device, as for example, in a lateral displacement from the central aperture in the pipe 23, but interiorly of the O-ring 24. By appropriate connection of these two thermistors in a conventional bridge circuit, for example, it will thus be seen that one of the thermistors is influenced as to the resistance thereof by the amount of air flowing through the device, while the remaining thermistor operates as a reference, and is not influenced by his air flow. Alternatively, the flowmeter may include three thermistors, for example, with all of them aligned in the air flow path to produce differential resistance variations in accordance with the volume and direction of air flow.

There is illustrated in FIGURE 3 a simple bridge circuit, wherein the two thermistors 26 and 27 of a flowmeter are included. A pair of input terminals 31 and 32 connected across a low-voltage source, as for example four and one-half volts, and the first thermistor 26 is connected in series with a resistor 33 across these terminals. A second resistor 34, identical to resistor 33, is connected in series with the other thermistor 27 in parallel with the first thermistor 26 and resistor 34 across the terminals 31 and 32. Current flow through the thermistors heats them. A current-responsive device or circuit, such as a milliameter 36, is connected between the two resistor-thermistor junctions. In the arrangement of FIGURE 3 with the thermistors 26 and 27 mounted as illustrated in FIGURE 2, for example, it will be appreciated that the total absence of air flow through the flowmeter will result in a balanced bridge condition wherein no current flows through the milliameter 36. The flow of air through the flowmeter 18 will cool the thermistor 26 and consequently change the resistance thereof so as to unbalance the bridge circuit, and consequently cause a current to flow through the milliameter 36. This is the normal operating condition of the air flow alarm system of the present invention, wherein the interior of a volume 12 is slightly pressurized or evacuated with access apertures closed. There is consequently normally produced a certain current flow through the device or circuit 36. This may be considered as a standard condition. Opening of the door 13 to the enclosure 11, for example, will produce a very material variation in the internal pressure of volume 12 and,

consequently, will markedly change the volume of air flowing through the fiowmeter 18. This variation in air How will consequently cause a change in the resistance of the thermistor 26 and, consequently, will vary the balance of the electrical bridge circuit so as to change the current flowing through the milliameter 36. One manner of actuating an alarm by the present invention is to provide electrical contacts or stops on the milliameter pointer, which thereby actuate some external circuit upon the current changing a predetermined amount from that normally associated with substantially sealed condition of the enclosure. The foregoing is schematically illustrated in FIGURE 3 by a dashed line extending from the milliameter 36 to an alarm circuit schematically illustrated at 37. A predetermined variation in the current through the ammeter of the bridge circuit causes an output signal which may itself actuate an alarm or may be amplified to produce an alarm signal or to operate a recording system. Instead of an indicating means, the current-responsive device 36 may comprise a transistor amplifier, for example, connected across a relay coil to open the relay contacts in a bell circuit upon predetermined variation in bridge unbalance.

There is illustrated in FIGURE 4 a typical graph of output signal vs. air fiow for a system in accordance with the present invention. At zero flow there will be zero output, as illustrated in FIGURE 4, and with flow in one direction, there will be produced an unbalance of the bridge to thereby produce a current flow in one direction. A reverse flow of air through the fiowmeter will produce an unbalance in the bridge circuit causing a reverse polarity signal. By appropriately choosing limits of acceptable current flow, it is thus possible to quite adequately protect the volume 12 by monitoring the variation in air fiow into or out of the volume. This is illustrated in FIGURE 4 by the marks 41 and 42 on the characteristic curve of signal output vs. air flow for positive air flow. These points on the graph are indicative of maximum current variation allowed by a particular system without the production of an external signal of sufiicient magnitude to actuate an alarm system. Similar points on the left-hand portion of the graph may be chosen for opposite air flow.

It is believed to be obvious that innumerable types of alarm systems may be employed as, for example, visual or sonic alarms at the location of the protected volume, or silent alarms with appropriate indications at some remote station. In the alternative, there may only be employed a recording system to provide a permanent record of the variations in air flow which are, in turn, indicative of accesses to the protected volume. Although the present invention is particularly applicable to the protection of valuables as, for example, in a showcase or vault, it is also equally applicable to the maintenance of a permanent record of times of entry to a protected volume. This may be employed in connection with a vault, for example, wherein it is expected that certain authorized personnel will from time to time enter the protected area. With the permanent record available from the present invention, it is, of course, possible then to verify authorized entry and identify unauthorized entry.

It is possible for certain difficulties to be encountered in the protection of enclosures subjected to variations in atmospheric pressure. Thus, gusts of wind, for example, may temporarily upset the pressure differential established by the air pump or blower of the present invention and, consequently, produce a false alarm in the configuration of the invention illustrated in FIGURE 1. This difficulty may be overcome in the manner illustrated in FIGURE 5, for example. Referring to FIGURE 5, it will be seen that an enclosure 51 to be protected is divided into two separate volumes 52 and 53 by an internal wall 54. The blower or air pump 16 is mounted within one of the internal volumes 53 to exhaust into the other volume 52. The fiowmeter 18 is mounted through the wall 54 so as to measure the flow of air between the volumes 52 and 53. a

In the embodiment of FIGURE 5, the volumes 52 and 53 are preferably made substantially equal and, consequently, about one-half of the internal volume of the internal enclosure 51 is pressurized and one-half is evacuated by the blower 16. With this substantially equal division of the internal volume of the enclosure, and the pressurization of one and evacuation of the other portions of the volume, it will be seen that variations in external air pressure operate equally upon each of the two internal volumes 52 and 53. Consequently, windgusts, indicated by the arrows 57, will operate to relatively evenly raise the pressure of both volumes so that pressure differential between the two volumes will not be affected. Note that the wall 54 preferably divides the side of the enclosure subject to the wind so that each volume 52 and 53 is equally affected. In addition, the fiowmeter is not subjected directly to these external pressure variations, as may be brought about by change in atmospheric conditions, i.e., neither the fiowmeter nor blower is directly exposed to the external atmosphere. In this embodiment of the invention, opening of a door or window to either of the volumes 52 or 53 will produce a marked change in the flow of air through the meter 18 and will consequently produce an alarm signal or actuate recording apparatus, as discussed above. It will thus be seen that the embodiment of the present invention illustrated in FIGURE 5 is particularly applicable to the protection of volumes within enclosures having walls exposed to the external atmosphere.

An additional manner of overcoming undesirable effects in ambient air pressure lines in the utilization of a reference volume in conjunction with the fiowmeter, and such a system is illustrated in FIGURE 6. Referring to FIGURE 6, there is shown an enclosure 61 having an internal volume 62 to be protected and various openings such as doors and windows leading into this volume. A blower 16 is mounted with an exhaust duct 17 extending through a wall of the enclosure 61 so as to partially evacuate the volume 62. In this instance, the fiowmeter 18 is not mounted directly through a wall of the enclosure, but instead, is connected through a restriction 63 in a tube etxending through a wall of the enclosure. In addition, there is provided a chamber 64 having a small tube 66 connected to the juncture of the flow meter and restriction. This chamber 64, which may have a relatively small capacity as of the order of five gallons, or the like, operates together with the restriction 63 to introduce an adequate time constant for protecting the fiowmeter from variations in an external air pressure.

Operation of the system of FIGURE 6 directly follows the operation of the system of FIGURE 1, for example, insofar as generalities are concerned. Thus, the blower 16 continuously operates to draw a small vacuum in the protected volume 62. The fiowmeter 18 is connected through restriction 63 from inside the enclosure to the exterior thereof, and consequently produces a signal which is a function of air flow from the outside of the enclosure to the interior thereof, as established by the pressure differential existing across the enclosure walls. The capacity of the reference chamber 64 and the amount of restriction in air flow provided by the restriction 63 is adjusted to be substantially equal to the time constant of the blower charging up the entire protected volume. Consequently, variations in external pressure which do not last longer than the amount of time it normally takes for the blower to initially evacuate the volume 62 will not affect the fiowmeter to produce a variation in output current from the bridge circuit associated therewith. The reference volume 64 operates somewhat as a reservoir or surge tank, and together with the restriction 63 then serves to prevent the fiowmeter from being affected by external air pressure variations of short duration. In this system also, opening of a door, or the like, to the volume 62 will produce a substantially instantaneous change in air flow through the flowmeter, so as to unbalance the bridge circuit in which the fiowmeter thermistors are connected. This unbalance is employed as set forth above to actuate either an alarm system or recording system, or both.

The flow alarm system of the present invention is extremely rapid acting, and tests have shown that relatively small openings of a door, or the like, to a protected room will produce an operable alarm signal in the order of a second or less. In order to fully utilize the rapidity of action of the system, there may be employed transistorized circuitry for amplification of the bridge unbalance signal, to thereby energize or de-energize a sensitive relay, for example, connected in a bell alarm circuit. It is, of course, necessary to employ a safe electrical energization for the blower or air pump employed in the system of the present invention, and also to insure continuity of energization of the detecting equipment, in order to guarantee a truly fail safe system. It is to be particularly noted, however, that the system itself is intrinsically fail safe, inasmuch as any type of tampering with the blower, power supply, ducts, or flowmeter, will produce a variation in the output signal to thereby actuate an alarm. Disconnection of the blower or blocking of the blower duct produces a change in the differential pressure existing between the interior and exterior of the protecetd enclosure, and this consequently varies the flow of air through the flowmete-r so that the indicating circuitry connected thereto is actuated, and in the illustrated example the bridge circuit is unbalanced.

In testing of the present invention on an enclosure having a floor area of about 1,000 square feet and relatively large leakage about some nine windows and one door leading thereto, there were employed a 250 watt centrifugal blower maintaining a pressure of about 0.025 inch of water in the configuration illustrated in FIGURE 1. It was found that pressure varied by a factor of ten to twenty when any window or door was opened about two inches. The fiowmeter and detecting circuit, together with a simple amplifier to increase the usable value of current variations in the bridge circuit, was found to be quite adequately sensitive. It Was furthermore determined that normal cracks or crevices, as may be found for example about conventional doors and windows, did not contribute an undue amount of leakage, if they were not over about one-quarter of an inch wide. The foregoing results from the fact that the pneumatic resistance of small apertures varies with the inverse fourth power of the diameter of the aperture, assuming viscous flow. Further testing has indicated that a considerably smaller blower may be employed than the one identified in the tests above, and furthermore, it has been determined that heating means of normal size for a volume under protection does not upset the alarm system hereof. Even though a heater operating Within a room will cause the air therein to expand, such expansion has been found to be sufiiciently slow for the fan or blower to maintain the normal pressure differential without difiiculty.

Although the present invention has been described in connection with particular preferred embodiments there of, it is not intended to limit the invention to the specific terms of the description or details of the illustrations, but instead, reference is made to the appended claims for a precise delineation of the true scope of this invention.

What is claimed is:

1. An alarm system for a substantially sealed enclosure comprising a fiowmeter disposed within said enclosure and having a tube extending through a wall of said enclosure for producing a signal responsive to air flow through said tube as established by a pressure differential across said wall, means defining a restriction in the tube of the flowmeter, means defining an air chamber connected between the fiowmeter and restriction for delaying flowmeter response to changes in air flow thereto, and pump means extending through said wall and producing a substantially steady pressure differential across said wall, whereby opening of the enclosure varies said pressure differential to vary the flowmeter signal as an indication of enclosure opening.

2. An air flow alarm system for a substantially sealed enclosure comprising a blower extending through a wall of the enclosure and operating to produce a pressure differential of at least 0.005 inch of water between the interior and exterior of the enclosure with access means closed, a flowmeter disposed within the enclosure and having an air flow tube extending through a wall of said enclosure to produce a signal responsive to amount of air flow therethrough, means defining a restriction in said tube between the outer end thereof and the flowmeter, means defining a small air chamber connected to said tube between said restriction and said fiowmeter with the capacity of said chamber and the air flow resistance of said restriction being proportioned to establish a flowmeter time delay of the order of time required for the blower to establish a substantially steady pressure difference across the wall at initial closure of the closure, and alarm means actuated by said flowmeter signal.

References Cited by the Examiner UNITED STATES PATENTS 623,689 4/1899 Smith 340-276 X 2,069,953 2/1937 Hopkins 340240 2,129,261 9/1938 Chase. 2,745,089 5/1956 Levy 340-285 2,828,479 3/1958 Jackson 340239 3,075,552 1/1963 Hunt 340-267 X FOREIGN PATENTS 749,230 5/1956 England. 189,772 6/ 1937 Switzerland.

THOMAS B. HABECKER, Acting Primary Examiner.

NEIL C. READ, Examiner. R. GOLDMAN, Assistant Examiner. 

1. AN ALARM SYSTEM FOR A SUBSTANTIALLY SEALED ENCLOSURE COMPRISING A FLOWMETER DISPOSED WITHIN SAID ENCLOSURE AND HAVING A TUBE EXTENDING THROUGH A WALL OF SAID ENCLOSURE FOR PRODUCING A SIGNAL RESPONSIVE TO AIR FLOW THROUGH SAID TUBE AS ESTABLISHING BY A PRESSURE DIFFERENTIAL ACROSS SAID WALL, MEANS DEFINING A RESTRICTION IN THE TUBE OF THE FLOWMETER, MEANS DEFINING AN AIR CHAMBER CONNECTED BETWEEN THE FLOWMETER AND RESTRICTION FOR DELAYING FLOWMETER RESPONSE TO CHANGES IN AIR FLOW THERETO, AND PUMP MEANS EXTENDING THROUGH SAID WALL AND PRODUCING A SUBSTANTIALLY STEADY PRESSURE DIFFERENTIAL ACROSS SAID WALL, WHEREBY OPENING OF THE ENCLOSURE VARIES SAID PRESSURE DIFFERENTIAL TO VARY THE FLOWMETER SIGNAL AS AN INDICATION OF ENCLOSURE OPENING. 